OpenRocket Cnalpha accuracy

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We did an impact force estimation and applied a 1.5 safety factor. Came out thicker than kits similarly sized to our rocket so we're taking that to mean we have margin on that front.

Sounds like your fins are either G10 or plywood if you're comparing them to a kit. Mighty excessive safety factor. Your "margin" on the front
will not help if the chute does not open or the size is inadequate on a heavy rocket. Be sure to focus on the recovery design if this is your
first competition; NASA SLI? Spaceport Cup?
 
That may be helpful for other users. In our case it was easy enough to re-create the rocket in RasAero, the problem is that RasAero isn't as open so we can't say to our department head that we know exactly how it runs. I have no reason to doubt RasAero's output, and the comparisons to wind tunnel data for RasAero look better but that's not quite enough.


RASAero II uses a combination of USAF DATCOM and Missile DATCOM methods, with some models based on data from Fluid Dynamic Drag by Hoener. Some recent modifications where specifically Missile DATCOM related.


I'm actually starting to write up some technical articles on the methods used in RASAero II. Unfortunately, I haven't published anything up to this point.


For background, I'm a 37-year experienced aerospace engineer, 22 years as a civilian with the Air Force, 15 years with NASA.


For wind tunnel comparisons, see the RASAero web site ( www.rasaero.com ). Also see the RASAero II CP prediction comparisons with Supersonic CP wind tunnel data for the ARCAS sounding rocket in the following Rocketry Forum thread:

http://www.rocketryforum.com/showth...h-Supersonic-CP-and-CD-ARCAS-Wind-Tunnel-Data

I haven't gotten around to putting the ARCAS comparison up on the RASAero web site.



Charles E. (Chuck) Rogers
Co-Author of the RASAero II Software
Rogers Aeroscience
 
RASAero II uses a combination of USAF DATCOM and Missile DATCOM methods, with some models based on data from Fluid Dynamic Drag by Hoener. Some recent modifications where specifically Missile DATCOM related.


I'm actually starting to write up some technical articles on the methods used in RASAero II. Unfortunately, I haven't published anything up to this point.


For background, I'm a 37-year experienced aerospace engineer, 22 years as a civilian with the Air Force, 15 years with NASA.


For wind tunnel comparisons, see the RASAero web site ( www.rasaero.com ). Also see the RASAero II CP prediction comparisons with Supersonic CP wind tunnel data for the ARCAS sounding rocket in the following Rocketry Forum thread:

http://www.rocketryforum.com/showth...h-Supersonic-CP-and-CD-ARCAS-Wind-Tunnel-Data

I haven't gotten around to putting the ARCAS comparison up on the RASAero web site.



Charles E. (Chuck) Rogers
Co-Author of the RASAero II Software
Rogers Aeroscience

Very cool, I can't wait to read more about it! And thanks for the additional data
-Craig
 
Historically Open Rocket has been good enough for sub sonic amateur rocketry. Supersonic fliers have always used RAS Aero as an add on to the initial design in Open Rocket. Perhaps getting your top brains in the team to participate in the OR development team could be a way forward. As your team and HOD need to understand how the analysis is done , it seems reasonable that you would understand the analysis and be able to contribute. I think it's unlikely that RAS Aero will go open source as the larger version forms a commercial product.
The new version of OR needs to be released soonish and it's unlikely to include all that you want/need for this next release. Maybe the next one with your teams assistance.
Norm


Currently RASAero II cannot import OpenRocket files. OpenRocket files can be converted to RockSim files, and then the RockSim file can be imported into RASAero II.


Note that RASAero II includes additional fin airfoils which are not included in OpenRocket or RockSim. The actual rocket fin airfoil may have been approximated by another airfoil in OpenRocket or RockSim. So after importing the file into RASAero II, the rocket fin airfoil data should be checked.

Below are the airfoils included in RASAero II:
Hexagonal
NACA
Double-Wedge
Biconvex
Hexagonal Blunt-Base
Single Wedge
Rounded
Square


Additionally, for the more accurate Rail Guide and Launch Shoe drag models in RASAero II, the Rail Guides and Launch Shoes inputs should be re-entered into RASAero II after the RockSim file has been imported.


While RASAero II is Free, it is not open source. I've begun working on technical articles documenting the methods used in RASAero II. So the methods will (eventually) be published.



Charles E. (Chuck) Rogers
Rogers Aeroscience
 
It's something I'd certainly like to see (see OR issue #875 :rolleyes: ). Is the RASaero file format documented?

The RASAero II .CDX1 input file format is not documented, but when you look at the input file, it is pretty self-explanatory. An excerpt is below:

<NoseCone>
<PartType>NoseCone</PartType>
<Length>34</Length>
<Diameter>6.125</Diameter>
<Shape>Tangent Ogive</Shape>
<BluntRadius>0.125</BluntRadius>
<Location>0</Location>
<Color>Black</Color>
</NoseCone>
<BodyTube>
<PartType>BodyTube</PartType>
<Length>10</Length>
<Diameter>6.125</Diameter>
<LaunchLugDiameter>0</LaunchLugDiameter>
<LaunchLugLength>0</LaunchLugLength>
<RailGuideDiameter>0</RailGuideDiameter>
<RailGuideHeight>0</RailGuideHeight>
<LaunchShoeArea>0</LaunchShoeArea>
<Location>34</Location>
<Color>Black</Color>
<BoattailLength>0</BoattailLength>
<BoattailRearDiameter>0</BoattailRearDiameter>
<BoattailOffset>0</BoattailOffset>
<Overhang>0</Overhang>
</BodyTube>



Charles E. (Chuck) Rogers
Rogers Aeroscience
 
Both of the papers here show the same discrepancy in simulated normal force coefficient vs. wind tunnel data. They're both rather old so if this has been addressed in later releases it'll save my team some work. https://openrocket.info/documentation.html


For the Subsonic descprency in CNalpha, run RASAero II using the Rogers Modified Barrowman Method.


The Barrowman method underpredicts the rocket CNalpha because:

1) The Barrowman Method assumes the Nose Cone CNalpha is 2.0. The RASAero II Rogers Modified Barrowman method includes the lift carry-over from the Nose Cone onto the Body Tube behind the Nose Cone. The combined Nose-Cone - Body Tube CNalpha is over 2.0, as a function of the Body Tube length.

2) The Barrowman Method leaves out the Body in the Presence of the Fin Interference Factor, it assumes it is zero. The RASAero II Rogers Modified Barrowman method includes the Body in the Presence of the Fin Interference Factor.


1) and 2) above, with the RASAero II Rogers Modified Barrowman Method, increases the CNalpha's at the Nose end and the Fin end of the rocket. The Center of Pressure (CP) doesn't change by much comparing the Barrowman Method and the Rogers Modified Barrowman Method, but the Barrowman Method underpredicts the rocket Total CNalpha.


For turning on the Rogers Modified Barrowman Method in RASAero II, see Pages 54-55 in the RASAero II Users Manual.



Charles E. (Chuck) Rogers
Rogers Aeroscience
 
For the Subsonic descprency in CNalpha, run RASAero II using the Rogers Modified Barrowman Method.


The Barrowman method underpredicts the rocket CNalpha because:

1) The Barrowman Method assumes the Nose Cone CNalpha is 2.0. The RASAero II Rogers Modified Barrowman method includes the lift carry-over from the Nose Cone onto the Body Tube behind the Nose Cone. The combined Nose-Cone - Body Tube CNalpha is over 2.0, as a function of the Body Tube length.

2) The Barrowman Method leaves out the Body in the Presence of the Fin Interference Factor, it assumes it is zero. The RASAero II Rogers Modified Barrowman method includes the Body in the Presence of the Fin Interference Factor.


1) and 2) above, with the RASAero II Rogers Modified Barrowman Method, increases the CNalpha's at the Nose end and the Fin end of the rocket. The Center of Pressure (CP) doesn't change by much comparing the Barrowman Method and the Rogers Modified Barrowman Method, but the Barrowman Method underpredicts the rocket Total CNalpha.


For turning on the Rogers Modified Barrowman Method in RASAero II, see Pages 54-55 in the RASAero II Users Manual.



Charles E. (Chuck) Rogers
Rogers Aeroscience
That is super helpful, thank you!

-Craig
 
Reading back I can't see what value you currently have for damping ratio (or coupled damping ratio) or what values you desire.

Before chasing too far down the rabbit hole of CNalpha computations, you need to perform a sensitivity analysis. Since CNalpha appears both in the numerator (as a weighted sum) and the denominator (square root of the total), it isn't going to be very sensitive to errors in a particular component.
 
Are you being this thorough with Fin Flutter? You have a greater chance of shredding your fins then having a rocket go unstable.
I don't know how you can say this without seeing the design of the rocket.
 
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